# std::inplace_merge

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class BidirIt > void inplace_merge( BidirIt first, BidirIt middle, BidirIt last ); | (1) | (constexpr since C++26)
template< class ExecutionPolicy, class BidirIt > void inplace_merge( ExecutionPolicy&& policy, BidirIt first, BidirIt middle, BidirIt last ); | (2) | (since C++17)
template< class BidirIt, class Compare > void inplace_merge( BidirIt first, BidirIt middle, BidirIt last, Compare comp ); | (3) | (constexpr since C++26)
template< class ExecutionPolicy, class BidirIt, class Compare > void inplace_merge( ExecutionPolicy&& policy, BidirIt first, BidirIt middle, BidirIt last, Compare comp ); | (4) | (since C++17)

Merges two consecutive sorted ranges `[`first`,` middle`)` and `[`middle`,` last`)` into one sorted range `[`first`,` last`)`.

1) If `[`first`,` middle`)` or `[`middle`,` last`)` is not sorted with respect to operator<(until C++20)std::less{}(since C++20), the behavior is undefined.
3) If `[`first`,` middle`)` or `[`middle`,` last`)` is not sorted with respect to comp, the behavior is undefined.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

This merge function is stable, which means that for equivalent elements in the original two ranges, the elements from the first range (preserving their original order) precede the elements from the second range (preserving their original order).

If any of the following conditions is satisfied, the behavior is undefined:

- `[`first`,` middle`)` or `[`middle`,` last`)` is not a valid range.

The type of *first is not Swappable. | (until C++11)
`BiditIt` is not ValueSwappable. The type of *first is not MoveConstructible. The type of *first is not MoveAssignable. | (since C++11)

### Parameters

first | - | the beginning of the first sorted range
middle | - | the end of the first sorted range and the beginning of the second
last | - | the end of the second sorted range
policy | - | the execution policy to use
comp | - | comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second. The signature of the comparison function should be equivalent to the following: bool cmp(const Type1& a, const Type2& b); While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, `Type1&` is not allowed, nor is `Type1` unless for `Type1` a move is equivalent to a copy(since C++11)). The types Type1 and Type2 must be such that an object of type BidirIt can be dereferenced and then implicitly converted to both of them. ​
Type requirements
- `BidirIt` must meet the requirements of LegacyBidirectionalIterator.
- `Compare` must meet the requirements of Compare.

### Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1) Exactly \(\scriptsize N-1\)N-1 comparisons using operator<(until C++20)std::less{}(since C++20) if enough additional memory is available, \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) comparisons otherwise.
2) \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) comparisons using operator<(until C++20)std::less{}(since C++20).
3) Exactly \(\scriptsize N-1\)N-1 applications of the comparison function comp if enough additional memory is available, \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) applications otherwise.
4) \(\scriptsize O(N \cdot \log(N))\)O(N⋅log(N)) applications of the comparison function comp.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

See the implementations in libstdc++ and libc++.

### Notes

This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_constexpr_algorithms` | `202306L` | (C++26) | constexpr inplace merging (1), (3)

### Example

The following code is an implementation of merge sort.

Run this code

    #include <algorithm>
    #include <iostream>
    #include <vector>

    template<class Iter>
    void merge_sort(Iter first, Iter last)
    {
        if (last - first > 1)
        {
            Iter middle = first + (last - first) / 2;
            merge_sort(first, middle);
            merge_sort(middle, last);
            std::inplace_merge(first, middle, last);
        }
    }

    int main()
    {
        std::vector<int> v{8, 2, -2, 0, 11, 11, 1, 7, 3};
        merge_sort(v.begin(), v.end());
        for (const auto& n : v)
            std::cout << n << ' ';
        std::cout << '\n';
    }

Output:

    -2 0 1 2 3 7 8 11 11

### See also

merge | merges two sorted ranges (function template)
sort | sorts a range into ascending order (function template)
stable_sort | sorts a range of elements while preserving order between equal elements (function template)
ranges::inplace_merge (C++20) | merges two ordered ranges in-place (algorithm function object)

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